Difference Between Incoloy and Inconel

Inconel and Incoloy are distinct families of nickel-based alloys that share some overlap yet serve different technical roles. Inconel alloys are nickel-rich superalloys engineered for very high strength and exceptional resistance to oxidation and corrosive attack at elevated temperatures; they usually contain greater than 50 percent nickel and additional strengthening elements such as niobium, molybdenum or cobalt. Incoloy alloys combine nickel, iron and chromium in a balance that gives excellent resistance to carburizing, nitriding and many corrosive chemical environments while keeping cost and thermal stability optimized for long service at moderate to high temperatures. For high-temperature structural applications and severe corrosive media, Inconel is typically chosen. For industrial furnaces, petrochemical heaters, and some chemical process equipment where carburization resistance and economy matter, Incoloy grades often make more sense.

1. Quick comparison snapshot

This snapshot reflects broad tendencies; for design use, consult specific datasheets.

2. Trademark and historical background

Both names function today as industry trademarks that refer to families of engineered nickel alloys with distinct development histories. The INCONEL brand originated with research into nickel-chromium alloys developed for oxidation and corrosion resistance in oxygenated, high temperature environments. The INCOLOY family originated from efforts to create nickel-iron-chromium alloys with balanced thermal stability and resistance to carburization and oxidation for furnaces and process equipment. Manufacturers, material suppliers and standards committees publish datasheets and technical bulletins that describe the chemistry, mechanical properties and recommended applications for each grade.

3 Representative grades and UNS numbers

Practically, engineers refer to particular grades rather than the family name alone. Representative selections:

• Inconel common grades

  • Inconel 625: UNS N06625. High strength and corrosion resistance with service range from cryogenic to about 982°C in many atmospheres.

  • Inconel 718: UNS N07718. Age-hardenable, high strength up to roughly 700°C; widely used in aerospace and high-stress parts.

  • Inconel C-276: UNS N10276. Exceptional corrosion resistance in highly oxidizing and reducing chemical media.

• Incoloy common grades

  • Incoloy 800 / 800H / 800HT: UNS N08800, N08810, N08811. Nickel-iron-chromium series designed for thermal stability and resistance to carburization at high temperature.

  • Incoloy 825: UNS N08825. Good resistance to acid attack and stress corrosion cracking in many process environments.

When comparing, always refer to the exact UNS or W.Nr for mechanical limits and chemistry.

4 Chemistry and alloying strategy

Different design goals create distinct chemistries. Below is a simplified composition table that compares typical, representative grades. Percentages are approximate limits drawn from manufacturer datasheets; consult the exact standard for procurement decisions.

Notes: exact limits vary by specification and product form. Inconel engineers rely on elements such as niobium and molybdenum to provide solid solution and precipitation strengthening and beneficial resistance to localized corrosion. Incoloy chemistries keep nickel lower and iron higher to balance cost, thermal expansion and performance in carburizing environments.

5. Microstructure and strengthening mechanisms

• Inconel precipitation-hardened grades like 718 depend on controlled heat treatment to produce gamma prime and gamma double prime precipitates that deliver high yield and creep strength. Solid solution elements such as Mo and Nb give high-temperature stability and pitting resistance. Non-precipitation grades such as 625 obtain strength from solid solution hardening and cold working.

• Incoloy alloys such as 800 series are generally austenitic and rely on solid solution and microstructural stability to resist sigma phase formation under prolonged exposure. 800H and 800HT are modified for improved creep resistance through control of carbon and small additions of titanium and aluminum. The microstructural stability of Incoloy at prolonged high temperature is a key advantage in furnace service.

Understanding microstructure is essential for specifying heat treatment, welding procedure and expected life in creep or cyclic thermal service.

6. Mechanical properties and temperature limits

Below are representative mechanical values for common commercial conditions. Values vary with product form, heat treatment and testing method.

Designers must reference the supplier datasheet and applicable code for specific allowable stresses at temperature.

7 Corrosion resistance and service environments

• General corrosion and pitting: Inconel grades with higher nickel and molybdenum typically show superior resistance in chloride and reducing acids. Inconel C-276 and 625 are noted for resistance to pitting and crevice corrosion in seawater and many acids.

• Carburizing and nitriding atmospheres: Incoloy 800 family performs well in carburizing and nitriding environments common in furnaces and some chemical processing, due to alloy balance that minimizes embrittling carbides and sigma phase. This makes Incoloy a frequent choice for heater tubes, muffles and other furnace components.

• Stress corrosion cracking: For certain chloride environments and high tensile stresses, nickel-rich Inconel may offer better resistance, but selection must reference test data and field history. Incoloy 825 has been specified for service in acid gas and chloride rich environments where immunity to localized corrosion and SCC is needed at moderate temperatures.

Material selection should be based on actual process chemistry, temperature, flow regime and expected mechanical stress.

8 Fabrication, welding, forming and joining

• Welding: Many Inconel and Incoloy grades weld readily with matched filler alloys. Inconel 718 requires controlled thermal cycles and postweld heat treatment to restore age-hardening; weld procedure specifications are critical for high stress parts. Inconel 625 and C-276 are generally easier to weld with good HAZ corrosion resistance.

• Forming: Incoloy 800 series has good cold forming characteristics and can be drawn or rolled into tubes and sheets. Inconel alloys that are precipitation-hardenable may be more limited for heavy cold forming in their hardened condition; solution anneal and aging sequences matter.

• Heat treatment: For Inconel 718, specific solution anneal and aging treatments yield maximum strength. Incoloy 800H and 800HT are essentially heat treated to stabilize grain structure and optimize creep strength rather than to produce precipitation hardening. Follow supplier technical bulletins for recommended cycles.

Good fabrication practice includes qualified weld procedures, control of interpass temperatures, and postfabrication stress relief where necessary.

9. Standards, specifications and test requirements

When specifying either family, include the correct standards and heat numbers. Typical reference documents include:

• Manufacturer technical bulletins and datasheets for the specific UNS.

• AMS and ASTM material specifications for pressure parts and aerospace hardware. Example: AMS 5666 for Inconel 625 data and procurement.

• National and international standards for testing, e.g. ASTM E8 for tensile, ASTM G48 for pitting and crevice corrosion evaluation, ASTM G28 for corrosion in specific media. Consult code committees for pressure vessel allowable stresses at temperature.

• Supplier certificates and mill test reports per EN 10204/3.1 or 3.2 are common in procurement.

Provide full UNS, material form and heat treatment in purchase orders to ensure correct material control.

10. Typical applications and industry case examples

• Inconel typical uses

  • Gas turbine and aero engine components where high temperature strength and oxidation resistance are mandatory.

  • Chemical process equipment handling strong acids, halide environments and oxidative media where pitting is a concern, e.g., Inconel C-276 in chemical reactors.

  • Marine and subsea systems requiring superior localized corrosion resistance.

• Incoloy typical uses

  • Furnace components, radiant tubes, and heat treating fixtures due to resistance to carburization and dimensional stability.

  • Heat exchangers and piping in petrochemical plants where balanced nickel content and good strength at temperature make Incoloy cost effective.

  • Some chemical process applications where cost and adequate corrosion resistance are key, for example Incoloy 825 in acid gas environments.

Case example: a refinery heater may use Incoloy 800HT tubes for long life under carburizing flue gas while choosing Inconel cladding or bolting where localized corrosion risk is high.

11 Selection criteria and decision matrix

When choosing between the two families, weigh the following factors:

• Process chemistry: Highly oxidizing or halide-rich streams favor high Ni and Mo Inconel grades. Carburizing or nitriding atmospheres favor Incoloy 800 series.

• Operating temperature: For extreme mechanical loads above ~600°C, Inconel precipitation hardened grades can offer higher creep strength. For long term thermal stability in the 600°C to 850°C range where carburization is an issue, Incoloy 800H/HT often wins.

• Mechanical loading: High static or cyclic loads under elevated temperature often justify Inconel 718 or similar high-strength alloys.

• Fabrication and welding: Consider whether matched filler, weld procedure and postweld heat treatment are practical.

• Cost and procurement: Nickel content drives raw material cost. Many projects balance lifecycle cost and maintenance intervals against initial price.

Decision matrix (simplified)

Always confirm with corrosion testing, supplier data and codes.

12. Supply, cost and procurement considerations

Nickel market price and availability affect alloy selection. Inconel grades with higher nickel and critical alloying elements such as niobium typically cost more per kilogram than Incoloy 800 series. For large volumes, suppliers will quote material with lead times that reflect demand and stock. Mill test reports and third party inspection should be specified for critical parts. For pressure equipment, require traceable certifications and compliance to AMS/ASTM standards where applicable.

13. Environmental, life cycle and repair considerations

Recycling nickel alloys is common but energy intensive. Repair procedures for components in the field commonly use matched filler alloys and qualified welding procedures. Surface treatments, cladding and coatings might extend life where base alloy selection alone is marginal. In certain chemical plants, lining or cladding carbon steel with Inconel fared better economically than fabricating the complete component of a superalloy. Evaluate full life cycle costs including downtime, spare parts and repairability.

14. Practical specification checklist for procurement

When you issue a purchase order include the following:

1. Exact UNS number and trade name (for example Inconel 625 — UNS N06625).

2. Product form and dimensions.

3. Heat treatment condition and hardness limits.

4. Required mechanical properties and test methods (ASTM E8 tensile, hardness).

5. Corrosion testing requirements if applicable (ASTM G48, API or client-specified tests).

6. Traceability and certificate level (EN 10204 3.1 or equivalent).

7. Welding filler specification and qualification requirements.

8. Inspection and acceptance criteria for surface condition and dimensions.

Providing complete information reduces risk of material mismatch and costly rework.

15 Frequently Asked Questions

1. Are Inconel and Incoloy interchangeable?
   No. Although they share nickel in their composition, they have different alloying balances and intended service envelopes. Substitute only with thorough property comparison and qualification testing.

2. Which alloy family resists carburization better?
   Incoloy 800 family was developed for carburizing and nitriding environments and typically performs better in such furnace conditions.

3. Which is stronger at elevated temperature?
   Certain Inconel grades, especially age-hardenable ones like 718, have higher high temperature strength and creep resistance when correctly heat treated.

4. Is Inconel more expensive than Incoloy?
   Generally yes, due to higher nickel content and strategic alloying elements. Price varies with market nickel prices.

5. Can I weld Inconel 625 to Incoloy 800?
   Welding dissimilar alloys is possible with correct filler selection and qualified procedures, but HAZ behavior and thermal expansion differences must be considered. Test welds and procedure qualification are recommended.

6. What standards should I reference for specification?
   Use manufacturer technical bulletins and industry specifications such as AMS, ASTM or EN standards plus UNS numbers for clarity. For Inconel 625, AMS 5666 is often referenced.

7. Which alloy is better in seawater?
   High nickel, molybdenum containing Inconel grades typically show superior resistance to pitting and crevice attack in chloride environments over many Incoloy grades. Field testing advised for specific use.

8. How important is postweld heat treatment for Inconel 718?
   Very important. 718 attains its strength from precipitates formed during controlled aging. Postweld thermal cycles and aging restore mechanical properties and reduce susceptibility to cracking.

9. Do Incoloy alloys resist stress corrosion cracking?
   Some Incoloy grades, like 825, offer good resistance in particular acid chloride environments. However stress, temperature and specific chemistry must be evaluated.

10. Should I choose cladding or full alloy construction?
    Cladding carbon steel with a thin layer of Inconel or Incoloy can be economical for some services. Choice depends on mechanical loads, corrosion severity and constructability. Consult lifecycle cost models and supplier guidance.

16 Final recommendations for engineers and procurement

1. Specify the exact UNS and product form in procurement documents.

2. Match alloy selection to process chemistry and operating temperature rather than brand familiarity.

3. Require mill test certificates and appropriate non-destructive inspection where safety or pressure containment is involved.

4. Where uncertain, commission small scale corrosion coupons or loop testing under real process conditions to validate selection. Manufacturer technical services can often assist.

Authoritative references

• INCONEL® alloy 625 technical bulletin — Special Metals Corporation
• INCOLOY® alloy 800 technical bulletin — Special Metals Corporation
• INCONEL® alloy 718 technical bulletin — Special Metals Corporation
• VDM Alloy 625 datasheet — VDM Metals